Magnetic properties and particle-size analysis of dust-storm samples collected in Lanzhou and Sydney

Measurements of magnetic properties and particle size were carried out for dust-storm samples collected from Sydney (Australia) on 23 September 2009 and from Lanzhou (China) on April 2010. The results show that the magnetic mineral content of the Lanzhou sample is much higher than that of the Sydney sample. Magnetite, maghemite and possibly hematite occur in the Lanzhou sample. In addition to these minerals, there is also goethite in the Sydney sample. The magnetic grainsize of the Sydney sample is finer than that of the Lanzhou sample. These magnetic property differences are probably related to the source materials and anthropogenic particles. As to particle-size distribution, the Sydney sample shows a wide and flat curve with four peaks, while the Lanzhou sample displays a narrow curve with three peaks. The multi-peak particle-size distribution curves for the dust-storm samples indicate that there are multiple dust sources. Furthermore, a comparison of particle size between the Lanzhou storm sample and Jiuzhoutai loess/paleosol samples was also carried out. The results show that an increase in the 1-10 μm component and decrease in the 10-20 μm component of the loess/paleosol samples are mainly caused by pedogenic process (i.e. physical and chemical weathering of unstable minerals), while the increase in the 0.02-1 μm component is principally attributed to the formation of new minerals and the weathering of unstable minerals. The similarity of particle-size distribution curves between dust-storm and loess/paleosol samples implies that modern storm events are a good analogue for eolian processes operating in the Quaternary Period when loess/paleosol layers formed.